there's something rather charmingly naive about Jonathan Aitken's belief that the government has been treated harshly by broadcasters like John Humphrys - it's like a man with a mild case of tonsillitis assuring you that the experience has given him a real insight into pain and death. If Aitken wants to know what politically biased broadcasting really sounds like, he should go to America and see what Bill Clinton has to put up with.

The Clinton presidency has seen an extraordinary rise in the importance of radio as a medium for political discussion. In particular, it has seen the rise of a new breed of talk-show hosts: populist, intelligent, combative and deeply conservative. Rush Limbaugh is the undisputed heavyweight champion, with a daily audience of over 20 million on more than 600 stations across America, special Rush Rooms in restaurants and bars, where people can enjoy a drink or a meal without missing out on the Limbaugh wisdom, and an army of adoring fans, "Dittoheads" (whatever Rush says, they say ditto). When he talks of the importance of making sure that schools teach the four Rs (that's Reading, Riting, 'Rithmetic and Rush), he's probably joking; but a sizeable percentage of his listeners would probably think that a good idea.

His show revolves around Clinton-bashing, starting each day with an announcement of how many more days America has to endure the Clinton administration ("The biggest crisis in American history"), but he's willing to have a go at any other "liberal" cause. Feminism is a particular bug-bear: Rush invented the word "feminazis", and his 35 Undeniable Truths of Life, some of which are heard on Channel 4's Naked News documentary next Thursday, include No. 24: "Feminism was established so as to allow unattractive women easier access to the mainstream of society."

There are plenty of Limbaugh wannabes. On his home station, WABC in New York, there's the notoriously homophobic and racist Bob Grant - sadly, the extracts from Grant's show that were filmed for Naked News make him seem relatively mild: you just see him nodding calmly as somebody else complains about black people, rather than wading in himself. Nick Kent, the film's producer, complains that "one of the problems of making the film is that these guys are conscious of the cameras in the studio". Grant was an especially frustrating case: "Somebody I knew who was driving along listening to him while we were in the studio couldn't understand why he was being so nice."

The wannabes also include - a measure of the perceived importance of talk radio - a number of political figures from the right, including recently declared presidential candidate Pat Buchanan. The most recent recruit is Oliver North, former marine colonel and failed senate candidate (one of his gimmicks is putting callers he doesn't like through a shredder, a reference to his brisk way with paperwork during the Arms to Iran affair); and there's his fellow-felon G. Gordon Liddy, who was sentenced to 21 years in jail for his part in the Watergate burglaries (though he served only four). Kent recalls filming Liddy at a conservative political rally where "he was really hailed as a hero for what he did in Watergate".

Limbaugh reigns supreme, though, and it's not hard to see why. Listening in from this side of the Atlantic, where political discourse is conducted in relatively civilised, consensual terms, Limbaugh's style comes across as both refreshing and shocking - roughly comparable to having your head shoved down a toilet first thing in the morning. Populism is married to formidable shrewdness and eloquence; in this respect, Limbaugh is a long way removed from the nearest British equivalent, Talk Radio UK's Caesar the Geezer, whose populism is founded on ingrained ignorance - during a discussion about the dangers of burning your mouth on a McDonald's fruit pie just after TRUK started, Caesar insisted that it was impossible to heat food up to 200 degrees centigrade, because it would simply burn up; the lukewarm, underdone concoctions that must get served up in the Geezer household don't bear thinking about.

Unsurprisingly, the Clinton administration hasn't found any effective way of combating Limbaugh's onslaughts. Nick Kent was stunned, when filming at the White House, to observe how feeble their response was to him - "Completely outgunned, they've got no idea how to fight back." One reason is bad organisation. But he suggests that Limbaugh has an inherent advantage: "The conservative message is easy to put across, the liberal message is ambiguous and saying `Things are much more complicated than you think'."

After making the film, Kent is deeply suspicious of the kind of political influence wielded by Limbaugh (he's now spoken of by some as presidential potential himself). Of course, you could argue that Limbaugh is doing no more than articulate what people already think, but Kent doesn't buy that idea: "Someone can feel unhappy with their lot, but when somebody articulates that and says `You're unhappy because Clinton is in the White House,' that has an effect." Nor is he comfortable with the notion that this is a genuinely democratic, participatory kind of broadcasting - all calls are filtered, and Limbaugh is legendary for his rudeness to people with whom he disagrees (again, not something that comes across in the film), so that "it's a very mediated kind of access, at best". Other critics accuse Limbaugh of playing fast and loose with the facts; since this is a one-man show, there's nobody to correct him.

Limbaugh himself has two defences against charges of political bias. The first is to say that he has no political function, that he's just an entertainer, "a harmless little fuzzball". The second is that he is providing a tiny counterweight to the liberal media establishment. This is disingenuous of him; as counterweights go, Limbaugh is anything but tiny. Aside from the fact that there's 220lbs of him to throw on to the scales, how much weight Limbaugh and his fellow talk-radio jocks are seen to carry was made clear after the recent Republican landslide in the mid- term congressional elections, when Newt Gingrich, the new Speaker, publicly thanked the talk-show hosts for making it possible.

In any case, from a European perspective, the idea that the American media are particularly liberal seems absurd. It's clear, though, that Limbaugh's complaints about the media have struck a chord with many Americans. Several of the Dittoheads interviewed on Naked News talk about their disgust with the mainstream media, with newspapers and television; only on radio, they say, can they get things from a perspective they appreciate. Clearly, they regard themselves as part of the "Silent Majority" (a phrase borrowed, you may think significantly, from Richard M. Nixon) that Limbaugh says he represents. The way Rush's constituency sees things, its voice is being constantly drowned out by a leftist clamour (rather like a Conservative MP being interviewed by John Humphrys), and Rush is their only way of making themselves heard. Hence the strident, brash broadcasting style, in contrast to which BBC interviewing techniques seem deferential, almost apologetic; and also, it has to be said, a lot less fun - except for the people in power.